Lithographic apparatus with a fluid combining unit and related device manufacturing method

ABSTRACT

A system for tuning the refractive index of immersion liquid in an immersion lithographic apparatus is disclosed. Two or more immersion liquids of different refractive index are mixed together in order to achieve a desired refractive index. Further, the fluids may be conditioned and treated to maintain optical characteristics.

FIELD

The present invention relates to a lithographic apparatus and a methodfor manufacturing a device.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.comprising part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

It has been proposed to immerse the substrate in the lithographicprojection apparatus in a liquid having a relatively high refractiveindex, e.g. water, so as to fill a space between the final element ofthe projection system and the substrate. The point of this is to enableimaging of smaller features since the exposure radiation will have ashorter wavelength in the liquid. The effect of the liquid allows one touse a projection system with a NA of greater than 1. Other immersionliquids have been proposed, including water with solid particles (e.g.quartz) suspended therein or hydrocarbon fluids.

However, submersing the substrate or substrate and substrate table in abath of liquid (see, for example, U.S. Pat. No. 4,509,852, herebyincorporated in its entirety by reference) means that there is a largebody of liquid that must be accelerated during a scanning exposure. Thisrequires additional or more powerful motors and turbulence in the liquidmay lead to undesirable and unpredictable effects.

One of the solutions proposed is for a liquid supply system to provideliquid on only a localized area of the substrate and in between thefinal element of the projection system and the substrate using a liquidsupply system (the substrate generally has a larger surface area thanthe final element of the projection system). One way which has beenproposed to arrange for this is disclosed in PCT patent applicationpublication no. WO 99/49504, hereby incorporated in its entirety byreference. As illustrated in FIGS. 2 and 3, liquid is supplied by atleast one inlet IN onto the substrate, preferably along the direction ofmovement of the substrate relative to the final element, and is removedby at least one outlet OUT after having passed under the projectionsystem. That is, as the substrate is scanned beneath the element in a −Xdirection, liquid is supplied at the +X side of the element and taken upat the −X side. FIG. 2 shows the arrangement schematically in whichliquid is supplied via inlet IN and is taken up on the other side of theelement by outlet OUT which is connected to a low pressure source. Inthe illustration of FIG. 2 the liquid is supplied along the direction ofmovement of the substrate relative to the final element, though thisdoes not need to be the case. Various orientations and numbers of in-and out-lets positioned around the final element are possible, oneexample is illustrated in FIG. 3 in which four sets of an inlet with anoutlet on either side are provided in a regular pattern around the finalelement.

Early immersion lithographic machines have used water as the liquidbetween the final element of the projection system and the substrate.Water has a refractive index of about 1.4 and is relatively inexpensive.The next generation of lithographic immersion apparatus will likely usea liquid with a refractive index higher than that of water. Onedifficulty with the use of an immersion liquid with a higher refractiveindex than that of water is that the exact refractive index may varyfrom manufacturer to manufacturer and/or from batch to batch. This maybe a particular problem because the projection system of thelithographic apparatus is designed to be optimized for use at a certainrefractive index.

SUMMARY

It is desirable, for example, to provide an immersion lithographicapparatus which is capable of being used with one or more immersionliquids from different sources.

According to an aspect of the invention, there is provided alithographic apparatus comprising:

a first liquid supply system configured to provide a first liquid with afirst refractive index;

a second liquid supply system configured to provide a second liquid witha second refractive index which is lower than the first refractiveindex; and

a combining unit configured to combine the first and second liquids toachieve a combined liquid with a refractive index closer to a desiredrefractive index than the first refractive index or the secondrefractive index.

According to an aspect of the invention, there is provided a devicemanufacturing method comprising:

combining a first liquid with a first refractive index with a secondliquid with a second refractive index which is lower than the firstrefractive index in order to achieve a combined liquid with a refractiveindex closer to a desired refractive index than the first refractiveindex or the second refractive index; and

projecting a patterned beam of radiation onto a substrate through thecombined liquid, which liquid is provided between the substrate and aprojection system used to project the patterned beam.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIGS. 2 and 3 depict a liquid supply system for use in a lithographicprojection apparatus;

FIG. 4 depicts another liquid supply system for use in a lithographicprojection apparatus;

FIG. 5 depicts, in cross-section, a further liquid supply system for usein a lithographic projection apparatus;

FIG. 6 illustrates schematically an embodiment of the invention;

FIG. 7 depicts, in cross-section, a further type of liquid supply systemwhich may be used with an embodiment of the invention;

FIG. 8 illustrates schematically a further embodiment of the invention;and

FIG. 9 illustrates schematically another embodiment of the invention.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus according to oneembodiment of the invention. The apparatus comprises:

-   -   an illumination system (illuminator) IL configured to condition        a radiation beam B (e.g. UV radiation or DUV radiation);    -   a support structure (e.g. a mask table) MT constructed to        support a patterning device (e.g. a mask) MA and connected to a        first positioner PM configured to accurately position the        patterning device in accordance with certain parameters;    -   a substrate table (e.g. a wafer table) WT constructed to hold a        substrate (e.g. a resist-coated wafer) W and connected to a        second positioner PW configured to accurately position the        substrate in accordance with certain parameters; and    -   a projection system (e.g. a refractive projection lens system)        PS, held by a frame RF, configured to project a pattern imparted        to the radiation beam B by patterning device MA onto a target        portion C (e.g. comprising one or more dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure holds the patterning device in a manner thatdepends on the orientation of the patterning device, the design of thelithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The support structure can use mechanical, vacuum, electrostatic or otherclamping techniques to hold the patterning device. The support structuremay be a frame or a table, for example, which may be fixed or movable asrequired. The support structure may ensure that the patterning device isat a desired position, for example with respect to the projectionsystem. Any use of the terms “reticle” or “mask” herein may beconsidered synonymous with the more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more support structures). In such“multiple stage” machines the additional tables and/or supportstructures may be used in parallel, or preparatory steps may be carriedout on one or more tables and/or support structures while one or moreother tables and/or support structures are being used for exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL maycomprise various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross-section.

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the support structure (e.g., mask table) MT, and ispatterned by the patterning device. Having traversed the patterningdevice MA, the radiation beam B passes through the projection system PS,which focuses the beam onto a target portion C of the substrate W. Withthe aid of the second positioner PW and position sensor IF (e.g. aninterferometric device, linear encoder or capacitive sensor), thesubstrate table WT can be moved accurately, e.g. so as to positiondifferent target portions C in the path of the radiation beam B.Similarly, the first positioner PM and another position sensor (which isnot explicitly depicted in FIG. 1) can be used to accurately positionthe patterning device MA with respect to the path of the radiation beamB, e.g. after mechanical retrieval from a mask library, or during ascan. In general, movement of the support structure MT may be realizedwith the aid of a long-stroke module (coarse positioning) and ashort-stroke module (fine positioning), which form part of the firstpositioner PM. Similarly, movement of the substrate table WT may berealized using a long-stroke module and a short-stroke module, whichform part of the second positioner PW. In the case of a stepper (asopposed to a scanner) the support structure MT may be connected to ashort-stroke actuator only, or may be fixed. Patterning device MA andsubstrate W may be aligned using patterning device alignment marks M1,M2 and substrate alignment marks P1, P2. Although the substratealignment marks as illustrated occupy dedicated target portions, theymay be located in spaces between target portions (these are known asscribe-lane alignment marks). Similarly, in situations in which morethan one die is provided on the patterning device MA, the patterningdevice alignment marks may be located between the dies.

The depicted apparatus could be used in one or more of the followingmodes:

1. In step mode, the support structure MT and the substrate table WT arekept essentially stationary, while an entire pattern imparted to theradiation beam is projected onto a target portion C at one time (i.e. asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the support structure MT and the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the supportstructure MT may be determined by the (de-)magnification and imagereversal characteristics of the projection system PS. In scan mode, themaximum size of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the support structure MT is kept essentiallystationary holding a programmable patterning device, and the substratetable WT is moved or scanned while a pattern imparted to the radiationbeam is projected onto a target portion C. In this mode, generally apulsed radiation source is employed and the programmable patterningdevice is updated as required after each movement of the substrate tableWT or in between successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

A further immersion lithography solution with a localized liquid supplysystem is shown in FIG. 4. Liquid is supplied by two groove inlets IN oneither side of the projection system PL and is removed by a plurality ofdiscrete outlets OUT arranged radially outwardly of the inlets IN. Theinlets IN and OUT can be arranged in a plate with a hole in its centerand through which the projection beam is projected. Liquid is suppliedby one groove inlet IN on one side of the projection system PL andremoved by a plurality of discrete outlets OUT on the other side of theprojection system PL, causing a flow of a thin film of liquid betweenthe projection system PL and the substrate W. The choice of whichcombination of inlet IN and outlets OUT to use can depend on thedirection of movement of the substrate W (the other combination of inletIN and outlets OUT being inactive).

Another immersion lithography solution with a localized liquid supplysystem solution which has been proposed is to provide the liquid supplysystem with a liquid confinement structure which extends along at leasta part of a boundary of the space between the final element of theprojection system and the substrate table. Such a solution isillustrated in FIG. 5. The liquid confinement structure is substantiallystationary relative to the projection system in the XY plane thoughthere may be some relative movement in the Z direction (in the directionof the optical axis). In an embodiment, a seal is formed between theliquid confinement structure and the surface of the substrate and may bea contactless seal such as a gas seal.

The liquid confinement structure 12 at least partly contains liquid inthe space 11 between a final element of the projection system PL and thesubstrate W. A contactless seal 16 to the substrate may be formed aroundthe image field of the projection system so that liquid is confinedwithin the space between the substrate surface and the final element ofthe projection system. The space is at least partly formed by the liquidconfinement structure 12 positioned below and surrounding the finalelement of the projection system PL. Liquid is brought into the spacebelow the projection system and within the liquid confinement structure12 by liquid inlet 13 and may be removed by liquid outlet 13. The liquidconfinement structure 12 may extend a little above the final element ofthe projection system and the liquid level rises above the final elementso that a buffer of liquid is provided. The liquid confinement structure12 has an inner periphery that at the upper end, in an embodiment,closely conforms to the shape of the projection system or the finalelement thereof and may, e.g., be round. At the bottom, the innerperiphery closely conforms to the shape of the image field, e.g.,rectangular though this need not be the case.

The liquid is contained in the space 11 by a gas seal 16 which, duringuse, is formed between the bottom of the liquid confinement structure 12and the surface of the substrate W. The gas seal is formed by gas, e.g.air or synthetic air but, in an embodiment, N₂ or another inert gas,provided under pressure via inlet 15 to the gap between liquidconfinement structure 12 and substrate and extracted via outlet 14. Theoverpressure on the gas inlet 15, vacuum level on the outlet 14 andgeometry of the gap are arranged so that there is a high-velocity gasflow inwards that confines the liquid. Those inlets/outlets may beannular grooves which surround the space 11 and the flow of gas 16 iseffective to contain the liquid in the space 11. Such a system isdisclosed in United States patent application publication no. US2004-0207824, hereby incorporated in its entirety by reference.

In European patent application publication no. EP 1420300 and UnitedStates patent application publication no. US 2004-0136494, each herebyincorporated in their entirety by reference, the idea of a twin or dualstage immersion lithography apparatus is disclosed. Such an apparatus isprovided with two tables for supporting a substrate. Levelingmeasurements are carried out with a table at a first position, withoutimmersion liquid, and exposure is carried out with a table at a secondposition, where immersion liquid is present. Alternatively, theapparatus has only one table.

An embodiment of the invention is particularly suited to immersionliquids such as the next generation higher refractive index liquids.These fluids are most likely to be hydrocarbon fluids such as decalin.Candidate fluids include IF131 and IF132 produced by Dupont, HIL-1 andHIL-2 produced by JSR or Delphi produced by Mitsui. Other candidates aremixtures of fluids with nano-particles suspended in them or acids suchas phosphoric acids. The difficulty with these high refractive indeximmersion liquids is that different batches or indeed different (or eventhe same) types of liquid from various manufacturers can vary inrefractive index. This can cause problems because the projection systemof a lithographic apparatus is designed for use with an immersion liquidwith a specific refractive index. Although it could be possible to placevery strict restrictions on the refractive index of the immersion liquidused, it would be desirable if there were several manufacturers fromwhom high refractive index immersion liquids are available because thiswould keep the cost down. Also, the index of refraction of a fluid maychange under laser irradiation. The ability to shift the index of afluid by the addition of a second fluid may allow an increase in thelifetime of a fluid.

An embodiment of the invention addresses this problem by including aunit in the apparatus which can adjust the refractive index of thesupplied immersion liquid in order to make its refractive index closerto the refractive index for which the projection system of the apparatushas been designed.

FIG. 6 illustrates schematically an apparatus according to an embodimentof the invention. Two immersion liquids are supplied. The firstimmersion liquid is supplied by a first liquid supply system 410. Thefirst immersion liquid has a high refractive index. It is this immersionliquid which can vary in precise refractive index because of variationsin refractive index between batches and/or differences in manufactureror indeed difference in composition.

A second immersion liquid is also provided by a second liquid supplysystem 420. This second immersion liquid has a refractive index lowerthan that of the first immersion liquid. Thus, by combining both of thefirst and second immersion liquids at combining unit 430, a combinedimmersion liquid can be achieved which has a refractive index lower thanthat of the first refractive index and higher than that of the secondrefractive index. The exact refractive index can be calculated from thefollowing equation:

$x = \frac{\mathbb{d}\eta}{\left( {\eta_{1} - \eta_{2}} \right)}$where x is the fraction of second immersion liquid, dη is the change inrefractive index of the first immersion liquid required, η₁ is therefractive index of the first immersion fluid and η₂ is the refractiveindex of the second immersion liquid.

Thus, it can be seen that by combining the first and second immersionliquids in specific proportions it is possible to adjust the refractiveindex of the combined immersion liquid to be closer to the refractiveindex for which the projection system PS has been optimized than eitherthe respective first or second refractive indices of the first andsecond immersion fluids.

The combined immersion liquid is provided to the liquid supply system IHwhich provides liquid between the projection system PS and the substrateW (and may be of the sort illustrated in, for example, FIG. 2-5 or 7)such that the beam B from the projection system PS passes through theimmersion liquid before impinging on the substrate.

The combined immersion liquid may then be provided to a recycling unit440 which separates the first immersion liquid from the second immersionliquid and provides the separated first immersion liquid back to thefirst liquid supply system 410 and the separated second immersion liquidto, for example, the second liquid supply system 420. There may also besome waste which is disposed of.

In order to mix correct amounts of first and second immersion liquids, afeedback loop may be used. For example, the refractive index of thefirst immersion liquid may be measured in real time by a refractiveindex measuring unit 450 upstream of the combining unit 430. Based onthe measurement of refractive index of the first immersion liquid, avalve 460 can be opened or closed such that an appropriate amount ofsecond immersion fluid is provided from the second liquid supply system420 to the mixing unit 430.

Alternatively or additionally, the refractive index of the combinedimmersion liquid could be measured downstream of the combining unit 430and this information be used to adjust the amount of second immersionliquid (or first immersion liquid) provided to the mixing unit 430.

A further feedback loop is possible by using measurements performed bythe projection system PS (e.g. results from a transmission image sensor(TIS), an ILIAS sensor, etc.).

It may not be possible to adjust the refractive index of the firstimmersion liquid by combining it with the second immersion liquidaccurately enough for the projection system PS. Two further measures arepossible either in combination or individually in order to finallyadjust the apparatus. The first of these is a temperature controller 470which varies the temperature of the combined immersion liquid which hasthe effect of varying the refractive index. Thus, minor variations inthe refractive index of the combined immersion liquid can be made byvarying its temperature in order to make the refractive index of thecombined immersion liquid even closer or exactly the desired refractiveindex. The temperature controller 470 can also work in a feedback waywith the same or different inputs as the inputs to the combining unit430 described above. The refractive index of the combined immersionliquid can be measured up or downstream of temperature controller 470.

If the apparatus is not capable of varying the refractive index of thecombined immersion liquid accurately enough in order to match therefractive index for which the projection system PS has been optimized,it is possible to account for minor differences between the desiredrefractive index and the actual refractive index of the combinedimmersion liquid using one or more Z manipulators 480 of thelithographic apparatus, which manipulator normally moves the verticalposition of one or more optical elements of the projection orillumination system (and/or the patterning device or substrate) toadjust aberrations. This method can operate for a maximum dη of about0.002.

In an embodiment, the refractive index of the first immersion liquidshould be as close as is possible to the desired refractive index buthas a refractive index higher than the desired refractive index. Therefractive index of the second immersion liquid should desirably be aslow as possible so that only small quantities of the second immersionliquid need to be added to the first immersion liquid (so that as highas possible a desired refractive index is achievable). In an embodiment,the refractive index of the second immersion liquid is lower than 1.4,or less than about 1.3. Thus, for example, if the first refractive indexis 1.64 and the desired refractive index is 1.635 and the refractiveindex of the second immersion fluid is 1.3, the combined immersionliquid will only require 1.47% of the second immersion liquid using theabove equation. In an embodiment, there may be an advantage to using twoimmersion liquids of similar (but different) refractive index. In thisway fine tuning of the refractive index of the combined immersion fluidis easier because the mixing quantities of the first and secondimmersion fluids are closer to 50/50.

Examples of types of second immersion liquid could be fluorinatedhydrocarbons (teflon oil, fomblin-types, Krytox-types, etc.). Suchliquids, as is typical of liquids with lower refractive indices, havelow radiation absorbance and mix well with high refractive index liquidssuch as those described above. Alternatives may be (fully) saturatedmonocyclic hydrocarbons such as cyclohexane or a slightly fluorinatedversion of decalin.

As will be clear, an embodiment of the invention is equally applicableto the use of more than two liquids, for example three or four liquidscould be combined in the appropriate proportions to achieve the desiredrefractive index.

A typical liquid supply system will be described with reference to FIG.7 for a better understanding of the embodiments described with respectto FIGS. 8 and 9. It will become clear that, in an immersionlithographic apparatus, immersion liquid which is removed from a spacebetween the projection system PS and the substrate W (where it is atleast partly contained by a liquid supply system IH) can be removedthrough several different paths. Depending upon the path through whichthe immersion liquid is removed from the space, it may be advantageousto treat that immersion liquid before recycling it back into the spaceon an individual basis according to the path it has followed. This isbecause the path it has followed may determine the likely contaminantswithin it so that liquid purification may be customized to thoseparticular contaminants, for example. As well as immersion liquid beingextracted through the liquid supply system (often in the form of abarrier member 12), immersion liquid may also or alternatively beremoved through the substrate table WT, in particular by removal ofliquid which seeps into a gap between the edge of a substrate W and thesubstrate table WT.

FIG. 7 illustrates a barrier member 12 which is part of a liquid supplysystem IH. The barrier member 12 extends around the periphery of thefinal element of the projection system PS such that the barrier member(which is sometimes called a seal member) is, for example, substantiallyannular in overall shape.

The function of the barrier member 12 is to at least partly maintain orconfine liquid in the space between the projection system PS and thesubstrate W so that the beam B may pass through the liquid. The toplevel of liquid is simply contained by the presence of the barriermember 12 and the level of liquid in the space is maintained such thatthe liquid does not overflow over the top of the barrier member 12. Aseal is provided between the bottom of the barrier member 12 and thesubstrate W. In FIG. 7 the seal is a contactless seal and is made up ofseveral components. Working radially outwardly from the optical axis ofthe projection system PS, there is provided a (optional) flow controlplate 50 which extends into the space (though not into the path of thebeam B) which helps maintain parallel flow of the immersion liquid outof inlet 20 across the space and then out through an outlet (notillustrated) opposite and at the same level as the inlet (so that theimmersion liquid flows across the space between the final element of theprojection system and the substrate). The flow control plate 50 has oneor more through holes 55 in it to reduce the resistance to movement inthe direction of the optical axis of the barrier member 12 relative tothe projection system PS and/or substrate W. Moving radially outwardlyalong the bottom of the barrier member 12 there is then provided aninlet 60 which provides a flow of liquid in a direction substantiallyparallel to the optical axis towards the substrate. This flow of liquidis used to help fill any gaps between the edge of the substrate W andthe substrate table WT which supports the substrate. If this gap is notfilled with liquid, bubbles may be more likely to be included in theliquid in the space between the projection system PS and the substrate Wwhen an edge of the substrate W passes under the barrier member 12. Thisis undesirable as it can lead to deterioration of the image quality.

Radially outwardly of the inlet 60 is a extractor assembly 70 configuredto extract liquid from between the barrier member 12 and the substrateW. The extractor 70 will be described in more detail below and formspart of the contactless seal which is created between the barrier member12 and the substrate W.

Radially outwardly of the extractor assembly is a recess 80 which isconnected through an outlet 82 to the atmosphere and via an inlet 84 toa low pressure source. Radially outwardly the recess 80 is a gas knife90. An arrangement of the extractor, recess and gas knife is disclosedin detail in United States patent application publication no. US2006-0158627, incorporated herein its entirety by reference. However, inthat document the arrangement of the extractor assembly is different.

The extractor assembly 70 comprises a liquid removal device or extractoror outlet 100 such as the one disclosed in United States patentapplication publication no. US 2006-0038968, incorporated herein itsentirety by reference. Any type of liquid extractor can be used. In anembodiment, the liquid removal device 100 comprises an outlet which iscovered in a porous material 110 which is used to separate liquid fromgas to enable single-liquid phase liquid extraction. A chamber 120downstream of the porous material 110 is maintained at a slight underpressure and is filled with liquid. The under pressure in the chamber120 is such that the meniscuses formed in the holes of the porousmaterial prevent ambient gas (e.g., air) being drawn into the chamber120 of the liquid removal device 100. However, when the porous material110 comes into contact with liquid there is no meniscus to restrict flowand the liquid can flow freely into the chamber 120 of the liquidremoval device 100. The porous material 110 extends radially inwardlyalong the barrier member 12 (as well as around the space) and its rateof extraction varies according to how much of the porous material 110 iscovered by liquid.

A plate 200 is provided between the liquid removal device 100 and thesubstrate W so that the function of liquid extraction and the functionof meniscus control can be separated from one another and the barriermember 12 can be optimized for each. The plate 200 is a divider or anyother element which has the function of splitting the space between theliquid removal device 100 and the substrate W into two channels, anupper channel 220 and a lower channel 230 wherein the upper channel 220is between the upper surface of the plate 200 and the liquid removaldevice 100 and the lower channel 230 is between the lower surface of theplate 200 and the substrate W. Each channel is open, at its radiallyinnermost end, to the space.

An under pressure can be applied in the upper channel 220, rather thanleaving it open to the atmosphere through, e.g., one or more breathingholes 250. In this way the upper channel 220 can be made wider.

Thus, with the plate 200, there are two meniscuses 310, 320. A firstmeniscus 310 is positioned above the plate 200 and extends between theporous material 110 and the top surface of the plate 200 and a secondmeniscus 320 which is positioned underneath the plate 200 and whichextends between the plate 200 and the substrate W. In this way, forexample, the extractor assembly 70 can be configured for control of thefirst meniscus for optimum extraction of liquid and for positionalcontrol of the second meniscus 320 such that the viscous drag length forthe second meniscus 320 is reduced. For example, the characteristics, inparticular of the plate 200, may be optimized to make it energeticallyfavorable for the meniscus 320 to remain adhered to the plate 200 suchthat the scan speed of the substrate W beneath the barrier member 10 canbe increased. Capillary forces acting on the second meniscus 320 areoutwards and are balanced by an under pressure in the liquid adjacentthe meniscus so that the meniscus stays still. Higher loading on themeniscus, for example by viscous drag and inertia, results in a loweringof the contact angle of the meniscus with the surface.

As noted above, one or more breathing holes 250 may be provided at theradially outward most end of the plate 200 such that the first meniscus310 is free to move inwardly and outwardly beneath the porous material110 so that the extraction rate of the liquid removal device 100 canvary according to how much of the porous material 110 is covered byliquid. As illustrated in FIG. 7 the second meniscus 320 is adhered to alower innermost edge of the plate 200. In FIG. 7, the innermost loweredge of the plate 200 is provided with a sharp edge so as to pin thesecond meniscus 320 in place.

Although not specifically illustrated in FIG. 7, the liquid supplysystem has a mechanism to deal with variations in the level of theliquid. This is so that liquid which builds up between the projectionsystem PS and the barrier member 12 can be dealt with and does notspill. Such a build-up of liquid might occur during relative movement ofthe barrier member 12 to a projection system PS described below. One wayof dealing with this liquid is to provide the barrier member 12 so thatit is very large so that there is hardly any pressure gradient over theperiphery of the barrier member 12 during movement of the barrier member12 relative to the projection system PS. In an alternative or additionalarrangement, liquid may be removed from the top of the barrier member 12using, for example, an extractor such as a single phase extractorsimilar to the extractor 110.

That said, it should be noted that one or more embodiments of theinvention is applicable to any type of liquid supply system IH.

It can be seen from the description of FIG. 7 that there are severalways in which immersion liquid is removed from the space between thefinal element of the projection system and the substrate. These includeimmersion liquid which flows across the space out of inlet 20 and intoan outlet opposite the inlet 20 (the outlet is not illustrated). Thisimmersion liquid may or may not be irradiated depending upon when thebeam B is activated. Immersion liquid is removed by the extractor 70 andthis immersion liquid is likely to be extracted as a single phase. Otherimmersion liquid which escapes the extractor 70 could be collected bythe recess 80 and gas (or fluid-inert gas) knife 90 combination. Anysuch immersion liquid extracted is likely to be a combination of liquidand gas. Finally, liquid is also likely to be removed from the spacethrough the substrate table WT from between the edge of the substrate Wand the substrate table WT. This is also likely to have a high amount ofgas. Liquid which has been in contact with a top surface of thesubstrate (i.e. the resist) may also be contaminated by leaching so thatliquid may be best treated in a particular way different to otherliquid, as described below.

It is possible to treat at least two of the sources of immersion liquidseparately in a recycling path or circuit and two embodiments aredescribed. It will be seen that the principles of an embodiment of theinvention can be applied in a recycling system. Different sources havedifferent levels of contamination and the more contamination, the moredifficult and expensive recycling becomes.

In the below embodiments, the liquid supply system is assumed to havethree main types of liquid removed. These are single phase immersionliquid which has been in contact with the liquid supply system and whichmay or may not have been irradiated; two phase immersion liquid whichhas been in contact with the liquid supply system (i.e. immersion liquidextracted by a gas knife extractor); and immersion liquid which has beenin contact with the substrate table WT and is likely to be two phase. Inthe Figures these flows are labeled 1006, 1004 and 1002 respectively.

In an embodiment, referring to FIG. 8, the liquid supply system IH isillustrated schematically as is the substrate table WT on which thesubstrate W is supported. The solid arrows show the various flow pathsof immersion liquid in the liquid circuit 1000. As can be seen, liquidis prepared in a liquid preparation module 1150 and supplied throughline 1050 to the liquid supply system IH. The liquid supply system IHfills the space between the projection system PS and the substrate Wwith the liquid.

In this and other embodiments, three types of immersion liquid are shownas being removed from that space though there may be less or more thanthree. The three types of liquid are the liquid 1002 which is extractedfrom the space through the substrate table WT, the liquid 1004 which isextracted from the space through, e.g., a gas knife extractor and theliquid 1006 which is extracted through, e.g., an outlet in the side ofthe barrier member 12. Each of these types of liquid has its ownparallel liquid treatment units 1102, 1104, 1106 in the recyclingsystem. The parallel liquid treatment units 1102, 1104, 1106 areoptimized to treat the respective flow of immersion liquid for the typesof contaminants likely to be present.

Thus, the parallel liquid treatment unit 1102, which treats theimmersion liquid 1002 from the substrate table WT, has a degassing unitto degas the immersion liquid which passes through it, and has apurifier to purify the immersion liquid. The purifier will be optimizedto purify immersion liquid which has come into contact with thesubstrate table WT. The parallel liquid treatment unit 1102 also has oneor more particle filters which are optimized to extract particles likelyto have contaminated the immersion liquid in the substrate table WT. Inthe parallel liquid treatment units, the particle filter(s) is forfairly coarse particles.

Equally the parallel liquid treatment unit 1104 for the liquid 1004,which exits through, for example, the gas knife extractor of the liquidsupply system, has a degassing unit, a purifier and one or more particlefilters. The purifier and one or more particle filters of the parallelliquid treatment unit 1104 will be optimized for immersion liquid whichhas been in contact with the liquid supply system IH (e.g., barriermember 12). The unit 1104 will be optimized to remove particles andpurify immersion liquid which has been acted on by, for example, a gasknife which may result in its own particular type of impurities andparticles.

As will be appreciated, the liquid 1002 may also have been in contactwith the liquid supply system IH and the liquid 1004 may have been incontact with the top surface of the substrate table WT.

Finally, the liquid 1006, which has simply passed across the space andis therefore likely to be removed from the space as a single phase, willbe treated by the liquid treatment unit 1106 which may not comprise adegassing unit (because there will likely be no gas in the liquidbecause there would have been no opportunity for gas to be introducedinto that liquid) but will comprise a purifier and one or more particlefilters optimized to remove particles which are likely to exist in theliquid supply system.

The three flows are illustrative only. There may be other flows, forexample a single phase flow extracted through an extractor between theliquid supply system IH and the substrate W, such as extractor 70.

The flows of liquid out of the parallel liquid treatment units 1102,1104, 1106 are combined by a fluid cycling integrator 1110 and suppliedfurther as flow 1010 to a container or buffer 1120 where the liquid isstored until it is used by the fluid preparation unit 1150. The fluidpreparation unit 1150 may itself comprise several units to treat theliquid prior to it being supplied to the liquid supply system IH. Thefluid preparation unit 1150 can be seen as a serial liquid treatmentunit in that all of the recycled immersion liquid will pass through itfrom the container 1120 via flow 1020. The fluid preparation unit 1150could contain a degassing unit, a temperature control unit, a flowcontrol unit and a refractive index control unit. In the embodimentillustrated in FIG. 8, the fluid preparation unit 1150 has a fineparticle filter unit for final filtration after the one or more coarsefilters of the parallel liquid treatment units 1102, 1104, 1106. Ofcourse any of these parts of the fluid preparation unit 1150 could bepositioned separately in the flow paths 1010 or 1020.

Also a particle counter could be present in the fluid preparation unit(or elsewhere in the circuit) which detects the number and/or size ofany particulate impurities in the immersion liquid. The system could bearranged such that if more than a desired number or greater than adesired size of particles is detected, the immersion liquid isautomatically routed via a particle filter, either a separate particlefilter or one which is part of another component such as a liquidtreatment unit 1102, 1104, 1106. This additional aspect is applicable toany of the described embodiments.

Elements of the fluid preparation unit 1150 can be controlled in afeed-back manner based on measurements taken at the substrate table WTusing sensors 1212 and 1214. Sensor 1212 could, for example, be awavefront sensor and sensor 1214 could be an intensity (absorption)sensor. Based on the measurement results of these sensors, the fluidpreparation unit 1150 and the rest of the lithographic apparatus couldbe controlled to achieve the correct wavefront position and dose. Thisis achieved through control signals 2212 and 2214. The final preparationunit 1150 could vary how the immersion fluid is prepared prior toentering the liquid supply system IH and thereby control the refractiveindex (e.g. by temperature variation). One or both of those sensorscould also be used in determining when it is necessary to renew theimmersion liquid in the circuit 1000. Obviously it is desirable toensure that the absorption remains below a pre-determined maximumacceptable level and that the refractive index remains stable and if notthat the refractive index is known so that the necessary opticalcorrections can be made. Alternatively or additionally, there could be aregular program in place for the periodical replacement of liquid in thecircuit 1000.

Parts of the circuit 1000 could be supplied with the main bulk of theimmersion lithographic apparatus and other parts, in particular theparallel treatment units, could be provided as a separate unit from thebulk of the immersion lithographic apparatus.

The apparatus of this and other embodiments may be part of a closedsystem or a partially closed system. This is in contrast to an opensystem in which immersion liquid which is removed from the lithographicapparatus is either disposed or is re-worked offline and laterre-supplied to the lithographic apparatus. In a closed system the liquidin the apparatus is continually recycled and the liquid is notreplenished in use with fresh liquid. It may be necessary to include twopaths through which the fluid may be recycled in a closed system (aswell as in a partially closed system) in case for some reason a part ofthe recycling system becomes inoperative. Thus, effectively there wouldbe one or more valves to divert the liquid from, for example, one ormore of the liquid treatment units 1102, 1104, 1106, fluid cyclingintegrator 1110, container 1120 and fluid preparation unit 1150 to aseparate circuit comprising the same components. The valve(s) may bepart of one or more of those devices or in the flow path before or afterone or more of those devices as appropriate. In a partially closedsystem, fresh liquid can be added (for example to the container 1120during operation of the recycling system). Liquid exiting the liquidsupply system IH or substrate table WT could be diverted to be disposedof or to be re-worked offline prior to being re-supplied to thecontainer 1120. Using this system new immersion liquid can be added intothe circuit 1000 without interruption of the flow of immersion liquid sothat new immersion liquid can be added without any downtime of the wholeapparatus.

In an embodiment, the apparatus has a further control signal 2115 fromthe fluid preparation unit 1150 to a pressure control unit 1115. Thisensures that the fluid preparation unit 1150 receives immersion liquidat the correct rate. This arrangement is particularly useful if thefluid recycling integrator 1110 and the parallel treatment units 1102,1104, 1106 are part of a separate machine to the remainder of thecircuit 1000.

Furthermore, two sensors 1216, 1218 are provided in the path 1020 fromthe container 1120 to the fluid preparation unit 1150. These sensorsmeasure the refractive index of the immersion liquid directly and/or theabsorption of the immersion liquid.

The outputs of the sensors 2216, 2218 are provided to the fluidrecycling integrator (and/or one or more of the parallel treatment units1102, 1104, 1106) so that any necessary adjustment can be made directlyto reach the desired refractive index and/or absorption of the immersionliquid entering the fluid preparation unit 1150. This extra controlprior to entry of the immersion liquid into the fluid preparation unit1150 may result in increased control of the properties of the immersionliquid entering the liquid supply system IH.

Thus, it can be seen that parallel treatment units 1102, 1104, 1106 canbe seen as liquid supply devices providing liquids with differentrefractive indices. Thus, the feedback 2216, 2218 from the sensors 1216,1218 can be seen to provide feedback to the fluid recycling integrator1110 such that a desired refractive index is achieved. The fluidpreparation unit 1150 can vary the temperature of the immersion liquid,for example based on the output of sensors 1212 and 1214 which aresensors associated with the projection system. Therefore, the embodimentof FIG. 8, despite appearing different to the embodiment of FIG. 6,works on the same principle.

Referring to FIG. 9, another embodiment is depicted that is the same asthe embodiment described with respect to FIG. 8 except as describedbelow.

In order to ease splitting of the parallel treatment units 1102, 1104,1106 from the remainder of the lithographic apparatus, in thisembodiment the parallel treatment units 1102, 1104, used to treatimmersion liquid which may have had gas incorporated in it, do not havedegassing units. Instead, separate degassing units 1014, 1012 areprovided in the immersion liquid paths 1002, 1004 prior to thoseimmersion liquid paths reaching their respective parallel treatmentunits 1102, 1104. In this way the degassing units, which can be quitecomplicated, can be formed as part of the lithographic apparatus therebyeasing the splitting up of the unit into two parts.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 248, 193, 157 or 126 nm). The term“lens”, where the context allows, may refer to any one or combination ofvarious types of optical components, including refractive and reflectiveoptical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein.

One or more embodiments of the invention may be applied to any immersionlithography apparatus, in particular, but not exclusively, those typesmentioned above and whether the immersion liquid is provided in the formof a bath or only on a localized surface area of the substrate. A liquidsupply system as contemplated herein should be broadly construed. Incertain embodiments, it may be a mechanism or combination of structuresthat provides a liquid to a space between the projection system and thesubstrate and/or substrate table. It may comprise a combination of oneor more structures, one or more liquid inlets, one or more gas inlets,one or more gas outlets, and/or one or more liquid outlets that provideliquid to the space. In an embodiment, a surface of the space may be aportion of the substrate and/or substrate table, or a surface of thespace may completely cover a surface of the substrate and/or substratetable, or the space may envelop the substrate and/or substrate table.The liquid supply system may optionally further include one or moreelements to control the position, quantity, quality, shape, flow rate orany other features of the liquid.

The immersion liquid used in the apparatus may have differentcompositions, according to the desired properties and the wavelength ofexposure radiation used. For an exposure wavelength of 193 nm, ultrapure water or water-based compositions may be used and for this reasonthe immersion liquid is sometimes referred to as water and water-relatedterms such as hydrophilic, hydrophobic, humidity, etc. may be used.

One or more features or aspects of one or more embodiments herein may beused in combination with or substituted for one or more features oraspects of one or more other embodiments herein.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1. A lithographic apparatus comprising: a first liquid supply systemconfigured to provide a first liquid with a first refractive indexgreater than water at equal conditions; a second liquid supply systemconfigured to provide a second liquid with a second refractive indexwhich is lower than the first refractive index, the second liquidcomprising one or more selected from the group: fluorinated hydrocarbon,fomblin, Krytox type oil, decalin, saturated monocyclic hydrocarbon,cyclohexane, fluorinated decalin or hydrocarbon; a combining unitconfigured to combine the first and second liquids to achieve a combinedliquid with a refractive index lower than the first refractive index andcloser to a desired refractive index than the first refractive index orthe second refractive index and configured to control the combining thefirst and second liquids such that the combined liquid comprises 10% orless of the second liquid; and a refractive index measuring unit tomeasure the refractive index of the first liquid, of the combinedliquid, or of both, wherein the combining unit is configured to achievethe refractive index of the combined liquid based on the measuredrefractive index of the first liquid, of the combined liquid, or ofboth.
 2. The lithographic apparatus of claim 1, further comprising: aprojection system configured to project a patterned radiation beam ontoa target portion of a substrate; and a liquid supply system configuredto provide the combined liquid to a space between the projection systemand a substrate.
 3. The lithographic apparatus of claim 1, wherein thecombining unit is configured to use feedback in order to determinequantities of the first and second liquids to achieve the desiredrefractive index.
 4. The lithographic apparatus of claim 3, wherein thefeedback is based on a measurement of the refractive index of the firstliquid.
 5. The lithographic apparatus of claim 3, wherein the feedbackis based on a measurement of the refractive index of the combinedliquid.
 6. The lithographic apparatus of claim 3, wherein the feedbackis based on measurements taken by a sensor of a projection system of theapparatus.
 7. The lithographic apparatus of claim 1, wherein thecombined liquid comprises between 30 and 70% of the first liquid.
 8. Thelithographic apparatus of claim 1, further comprising a Z controllerconfigured to control a Z manipulator of a projection system tocompensate for any difference between the refractive index of thecombined liquid and the desired refractive index, the projection systemconfigured to project a patterned beam onto a target portion of asubstrate.
 9. The lithographic apparatus of claim 1, wherein thecombining unit is configured to achieve a combined liquid with arefractive index within 0.002 of the desired refractive index.
 10. Thelithographic apparatus of claim 1, further comprising a temperaturecontroller configured to vary a temperature of the combined liquid tochange the refractive index of the combined liquid towards the desiredrefractive index.
 11. The lithographic apparatus of claim 1, furthercomprising a fluid recycling system configured to separate the first andsecond liquids from the combined liquid after use and to return firstliquid to the first liquid supply system.
 12. The lithographic apparatusof claim 1, wherein the first refractive index is closer to the desiredrefractive index than the second refractive index.
 13. The lithographicapparatus of claim 1, further comprising a liquid conditioner configuredto condition the combined liquid to vary its refractive index.
 14. Adevice manufacturing method comprising: combining a first liquid with afirst refractive index greater than water at equal conditions with asecond liquid with a second refractive index which is lower than thefirst refractive index in order to achieve a combined liquid with arefractive index lower than the first refractive index and closer to adesired refractive index than the first refractive index or the secondrefractive index, the second liquid comprising one or more selected fromthe group: fluorinated hydrocarbon, fomblin, Krytox type oil, decalin,saturated monocyclic hydrocarbon, cyclohexane, fluorinated decalin orhydrocarbon and the combined liquid comprising 10% or less of the secondliquid; measuring the refractive index of the first liquid, of thecombined liquid, or of both, wherein the combining is performed toachieve the refractive index of the combined liquid based on themeasured refractive index of the first liquid, of the combined liquid,or of both; and projecting a patterned beam of radiation onto asubstrate through the combined liquid, which liquid is provided betweenthe substrate and a projection system used to project the patternedbeam.
 15. The method of claim 14, further comprising measuring therefractive index of (i) the first liquid, or (ii) of the second liquid,or (iii) of the combined liquid, or (iv) of any combination of(i)-(iii), and varying the fraction of the first liquid combined withthe second liquid to more closely achieve the desired refractive index.16. The method of claim 15, wherein the measuring includes measuring therefractive index of the combined liquid.
 17. The method of claim 15,wherein the measuring is performed by a sensor of the projection system.18. The method of claim 14, further comprising using a Z manipulator ofthe projection system to compensate for any difference between therefractive index of the combined liquid and the desired refractiveindex.